nand: mxc: Prepare to add support for i.MX5

Add some abstraction to NFC definitions so that some parts of the current code
can also be used for future i.MX5 code.

Clean up a few things by the way.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>